18 August 2010

Dude looks like a lady? Male lizards courting males

Oh, mistaken gender identity. Are you ever not funny?

Sudden change creates uncertainty. This is as true in evolution as it is in financial crises. Evolutionarily speaking, eastern fence lizards (Sceloporus undulatus) are in a moment of sudden and fast change – some of them, at least.

These lizards find themselves in a situation rather like some mice I’ve talked about before (here and here). A couple of thousand years ago, a new habitat opens up with very light coloured sand. Humans, in a fit of cleverness a few thousand years later, name it White Sands, New Mexico. The lizards see an opportunity and move in. For those that do, the darkest animals tend to have a hard time of it, because their high contrast make them easy pickings for predators. Lightly coloured animals get the advantage.

The rest of the population, though, in a more normal habitat, are being not picked on for having a darker hue. So the colour of the individual in the two habitats get more and more different over time.

What happens when you put members from these two divergent populations together in the same room? Will there be any behavioural differences, or will it just be as though one lizard thinks, “Oh, my brother, though we are long separated by the shifting sands, yet I still recognize you as a member of my kind, my tribe, my species!”

It didn’t quite work out that way. When they put together two males, a male from the White Sands habitat tended to react to a darkly coloured male from another habitat with:

“Whoa. Who’s the hottie?!”

The light coloured males gave courtship displays to those dark coloured males more than half the time. This is not usual behaviour: more typically, males who get introduced to another male will usually react aggressively, because these guys are territorial.

Although the colour of the lizards’ back is the most obvious cause to point to, the authors say it’s probably not the major difference between the populations that is driving this odd behaviour. When animals are displaying to each other, they do so in such a way as to show their undersides, not their backs.

The colouring of the lizards’ undersides also varies between the white sands and the dark soils. Lizards from the white sands tend to have bigger display patches on their undersides, and this is true for both males and females. Because the display patches of females from the white sands are larger, they tend to be fairly close in size to those of the males from the dark soil environments.

Extract from Figure 4. WS = white sands, DS = dark soil.

While this seems a plausible explanation, it is an untested hypothesis at this point. And it’s not clear right why larger patches might have had a selective advantage in a light sand environment.

But you could see how anyone could make that sort of mistake. Right? Right!?